J. Phys. Chem. B 2010, 114, 6675–6684
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Thermal Denaturation of Beta-Lactoglobulin and Stabilization Mechanism by Trehalose Analyzed from Raman Spectroscopy Investigations Jeong-Ah Seo,†,‡ Alain He´doux,*,† Yannick Guinet,† Laurent Paccou,† Fre´de´ric Affouard,† Adrien Lerbret,† and Marc Descamps† Unite´ Mate´riaux Et Transformations, UMR CNRS 8207, UniVersite´ de Lille 1, 59655 VilleneuVe d Ascq Ce´dex, France, and School of Engineering and Applied Sciences, HarVard UniVersity, Cambridge, Massachusetts 02138 ReceiVed: January 21, 2010; ReVised Manuscript ReceiVed: April 8, 2010
The thermal denaturation process of beta-lactoglobulin has been analyzed in the 20-100 °C temperature range by Raman spectroscopy experiments simultaneously performed in the region of amide modes (800-1800 cm-1) and in the low-frequency range (10-350 cm-1). The analysis of amide modes reveals a two-step thermal denaturation process in the investigated temperature range. The first step corresponds to the dissociation of dimers associated with an increase of flexibility of the tertiary structure. In the second step, large conformational changes are detected in the secondary structure and described as a loss of R-helix structures and a concomitant formation of β-sheets. Raman investigations in the low-frequency range provide important information on the origin of the denaturation process through the analysis of the solvent dynamics and its coupling with that of the protein. The softening of the tetrahedral structure of water induces the dissociation of dimers and makes the tertiary structure softer, leading to the water penetration in the protein interior. The methodology based on Raman investigations of amide modes and in the low-frequency region was used to analyze the mechanism of beta-lactoglobulin thermostabilization by trehalose. The main effect of trehalose is determined to be related to its capabilities to distort the tetrahedral organization of water molecules. I. Introduction The exceptional properties of trehalose to stabilize biomolecular systems are now well recognized,1,2 and widely used to protect proteins from the stresses that arise during purification or freeze-drying procedures induced by freezing or dehydration.3-5 The numerous hypotheses6-12 suggested for understanding the bioprotection mechanisms are generally suitable for narrow temperature and hydration ranges, that prevent a complete description of these mechanisms. As a consequence, the choice of the solutes used in freeze-drying processes and the development of stable formulations are still largely empirical. Recent investigations carried out on lysozyme13-15 and bovine serum albumin16 thermal denaturations have pointed out a common mechanism of bioprotection by disaccharides. A detailed analysis on lysozyme14 has clearly shown that trehalose was the most efficient bioprotectant to preserve the native conformation of the protein against high temperatures. The combination of Raman investigations14,15 and molecular dynamics simulations17 has shown that the mechanism of lysozyme thermostabilization by disaccharides was the result of two complementary effects. First, sugars have a destructuring effect on the tetrabonded H-bond network of water, in agreement with analyses performed on water/sugar solutions.9,17,18 As a consequence, O-H intermolecular interactions in the H-bond network of water are strengthened, leading to the stabilization of the tertiary structure of lysozyme. Second, sugars are excluded from the protein surface, and preserve the hydration shell of the protein, in line with the preferential hydration hypothesis.12 * Corresponding author. E-mail:
[email protected]. Phone: +33 320434677. Fax: +33 320436857. † Universite´ de Lille 1. ‡ Harvard University.
However, experimental evidence of protein thermostabilization is correlated to the protein denaturation process and then highly dependent on the protein structure. The results obtained on lysozyme were confirmed by a similar analysis on bovine serum albumin,16 which has similar structural characteristics. To get a general description of the bioprotection mechanism in the high temperature range, the influence of bioprotectants on the thermal denaturation of different proteins must be investigated. The secondary structure of bovine serum albumin19,20 (BSA) is characterized by a high R-helix content (∼51%) and quasi none β structure, while lysozyme19,20 is composed of R-helices (41%) and β-sheets (21%). In contrast, β-lactoglobulin (BLG) is mainly composed of β-sheets (54%) and R-helices (17%).21 At native pH, this protein of 162 residues is found in a dimeric conformation, while, at acidic pH, it dissociates into monomers, recognized as more stable than the native state.22,23 BLG is a globular protein widely analyzed for investigation of heatinduced aggregation.24-26 The thermal denaturation process of BLG was described as a multistep mechanism and highly dependent on the protein concentration and pH.27-29 At neutral and alkaline pH values and physiological concentrations (TH1) and TH1 in the spectral range 1650-1700 cm-1, plotted in Figure 4 in BLG solutions without trehalose, and previously associated with the formation of β-sheet structure. The plot of this integrated intensity against temperature for BLG solutions in the absence and presence of trehalose is reported in Figure 11b. This figure reveals that addition of trehalose induced a shift of the second step of denaturation toward higher temperatures, as previously observed for the first step of denaturation. Above 20% trehalose, the intensity variations at frequencies assigned to R-helices (intensity decrease around 1645 cm-1) and β-sheets (intensity increase around 1665 cm-1) become very weak, leading to large error bars and change in the shape of the fitting curve. Indeed, the sigmoidal shape of the transformation observed for trehalose concentration lower than 30% becomes sudden and discontinuous for 30 and 40% trehalose. The spectrum of the amide III region plotted in Figure 10b for various trehalose concentrations confirms that the structural changes become very weak with addition of trehalose, and quasi undetectable for 40 wt % trehalose. The amide III band region shows a strong increase of the intensity of the band corresponding to the β-sheets (3′b) and a significant decrease of the intensity of the bands corresponding to R-helices (4), in BLG solutions without trehalose. In the presence of trehalose ( 60 °C). Consequently, 30 wt % appears as a threshold concentration for the distortion of the hydrogen bond network of water and then for the stabilization of the organization of water molecules, in line to the study of the BLG thermostabilization by trehalose in the amide band region. IV. Concluding Remarks The mechanism of BLG thermal denaturation (for BLG (10 wt %) dissolved in D2O at pD 6.9) can be carefully detailed from Raman investigations carried out simultaneously in the 800-1800
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6683 cm-1 region of the amide bands and in the 10-350 cm-1 lowfrequency range. In the temperature range from room temperature up to 100 °C, BLG denaturation can be described as a two-step process. It is worth noting that additional structural changes could occur above 100 °C, as suggested in several works.46,47 The first step corresponds to the dissociation of dimers associated with an increase of flexibility of the tertiary structure, and then can be interpreted as the transformation of the native state into the socalled “molten globule” state. In the second step of the denaturation, conformational changes are detected and described as a loss of R-helix structures and a concomitant formation of β-sheets. The low-frequency analysis indicates that the softening and the breakdown of the hydrogen bond network are responsible for the dissociation of dimers and then the penetration of D2O in the monomer, leading to the destabilization of the secondary structure. The investigation of the low-frequency Raman susceptibility gives the unique opportunity to detect the dissociation of the dimers through the analysis of the low-frequency and predominant band (I) in the χ′′(ν) spectrum, reflecting the protein dynamics and the coupling of the protein and solvent dynamics. The enhancement of the intensity on the high-frequency side of the band (I), assigned to strong interactions between D2O and polar side chains of the protein, leads to a shift of the band (I) toward the high frequencies. No direct signature of the dimer dissociation has been evidenced at higher frequencies, in the spectral range corresponding to the molecular fingerprint of the protein. The examination of Figure 9 indicates that the dimer dissociation occurs progressively, as the transformation of the hydrogen bond network, by heating from room temperature up to 60 °C. At higher temperatures (T ) 80 °C), the transformation of the tertiary structure in a more flexible structure and the loss of R-helix structures favor additional protein-D2O interactions within the tertiary structure. The consequence is the enhancement of χ′′(ν) intensity on the low-frequency side of the band (I), reflecting the detection of new soft interactions between the solvent and the core of the protein, mainly composed of hydrophobic residues. Consequently, investigations in the low-frequency χ′′(ν) spectrum give information on the dynamics of the solvent, via the analysis of the band (II), which controls the motions of the protein through the coupling of the protein and the solvent dynamics, probed from the analysis of the shape of the band (I). The analysis of amide modes gives direct information on molecular conformational changes in the secondary structure, and also indirect information on the tertiary structure through the observation of isotopic exchanges between the protein and the solvent. By heating BLG solutions from room temperature up to 60 °C, only isotopic exchanges are detected through spectral modifications in the amide I and III regions. These changes have been assigned to the dissociation of dimers and to the penetration of the solvent in the protein interior, only possible in a highly flexible tertiary structure. By monitoring these amide modes, modifications of the secondary structure are detected with further heating of BLG solutions above TH1. The spectral changes observed near 960 and 1320 cm-1 in Figure 3a and near 1645 cm-1 in Figure 4 clearly reflect the loss of R-structures, while a concomitant formation of β-sheets is detected through the intensity increase of the Raman bands corresponding to amide mode III′ (980 cm-1) and amide mode I (1660 cm-1). The main effect of trehalose on BLG solutions primarily corresponds to modifications in the shape of the intermolecular O-D stretching band (II) in the χ′′(ν) spectrum, reflecting a transformation of the tetrahedral structure of water into a stiffened network. No significant change in the shape of the low-frequency band (I) is detected, in line with the hypothesis predicting the
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preferential exclusion of trehalose from the protein surface.1,12 Trehalose stabilizes the quaternary structure and also makes the tertiary structure more rigid and then more stable upon heating, in agreement with other investigations.48 A threshold concentration of 30 wt % was determined for the action of trehalose on the properties of the hydrogen bond network of water, which is related to the mechanism of BLG thermostabilization. Below 30%, the presence of trehalose induces a mere shift of the denaturation curves toward high temperatures, while, in the presence of 30% trehalose, partial thermal denaturation is observed in Figure 10b. At the highest concentration (40 wt %), isotopic exchanges are very weak (see Figure 10a, where the intensity of the 1240 cm-1 band remains nearly constant) and no spectral change can be detected in Figure 10b, which suggests no conformational change of BLG. However, the careful analysis of the amide I band reveals a very slight transformation of the secondary structure, as reported in Figure 11b. Consequently, a hierarchical stabilization of the protein structures is observed for the highest trehalose concentration. The native secondary structure is quasi preserved, because the solvent penetration in the protein interior is limited by the stabilization of the tertiary and quaternary structures, which arises from the stabilization of the distorted hydrogen bond network of water. BLG is characterized by a secondary structure mainly composed of β-sheets and the existence of a quaternary structure corresponding to dimer associations. The present work can be compared to the results obtained on other proteins characterized by different molecular weights and structural properties. Similar investigations have been carried out on lysozyme14 (14.3 kDa), composed of R-helix structures (41%) and β-sheets (21%), and bovine serum albumin16 (66 kDa), mainly composed of R-helices (51%). The main effect of trehalose on protein solutions, commonly observed for these proteins, is the transformation of the tetrahedral organization of water into a stiffened hydrogen bond network. The consequence is the stabilization of the distorted hydrogen bond network of water and then the stabilization of the most important structural edifice of the protein, i.e., the tertiary structure for lysozyme and albumin and the quaternary structure for β-lactoglobulin. Another common feature observed in these analyses is the reduction of the exposition of protein residues to the solvent in the presence of trehalose. However, this observation concerns different kinds of residues depending on the structural properties of the protein. For lysozyme characterized by a low molecular weight, trehalose is only preventing the solvent accessibility of buried residues in the tertiary structure during heating above 60 °C, while in albumin (characterized by a high molecular weight) a reduction of the exposition of hydrophilic side-chain residues around the protein surface is detected at room temperature. For β-lactoglobulin, trehalose preserves the dimer association upon heating and then limits the exposition of side-chain residues on the surface of monomers, and also limits the accessibility of the buried residues in the tertiary structure to the solvent, upon further heating. These common features induced by trehalose on different kinds of proteins dissolved in water are probably connected to the ability of trehalose to distort the tetrahedral structure of water and then to its exceptional capabilities to form hydrogen bonds with water, previously analyzed in recent works.17,42,49 Acknowledgment. This work was supported by the ANR (Agence Nationale de la Recherche) through the BIOSTAB project (“Physique Chimie du Vivant” program), by FEDER and by Nord-Pas de Calais region. References and Notes (1) Xie, G.; Timasheff, S. N. Biophys. Chem. 1997, 64, 25. (2) Kaushik, J. K.; Bhat, R. J. Biol. Chem. 2003, 278, 26458.
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